The canopy structure of grasslands is a major determinant of their use value, as it affects the quantity and the quality of the removed forage when mowed or grazed. The individual plant architecture determines the structure of the sward, and is highly sensitive to variations of the environment and to management practices. Several authors have suggested that this plasticity is partly mediated by a self-regulation process of the architecture. For instance, the height of the pseudostem could regulate the length of the leaves growing within it. To test this hypothesis we built a functional-structural 3D model of a ryegrass plant. The model uses the Lindenmayer-systems formalism (description of an object as a set of sub-units) and is based on the implementation of self-regulation rules. It satisfactorily captures the main static and dynamic architectural traits of ryegrass in non-limiting environmental conditions, so that a self-regulation process seems plausible. This model also allows testing the effects of cutting on the subsequent architecture development. This approach should ultimately allow us to better understand the morphogenetic variations and the evolution of the genotypic composition of simulated heterogeneous mono-species populations under changing environments.